The highly conserved co-factor Mediator Complex, comprising up to 30 subunits, serves as a molecular bridge between transcriptional activators and the core transcription machinery, and is thought to be required for most RNA polymerase II-dependent transcription. As the only known enzymatic subunit of Mediator complex, CDK8 (Cyclin-Dependent Kinase 8) can either positively or negatively regulate transcription. However, whether CDK8 acts as an activator or repressor at every gene, or whether it acts differently at different classes of genes, is a key unexplored field that would dramatically refine our view of how Mediator facilitates transcription. Furthermore, understanding the function and regulation of CDK8 is critical to elucidate how dysregulation of both CDK8 and its regulatory partner CycC (Cyclin C) contributes to a variety of human cancers. Depletion of CDK8 effectively blocks the growth of melanoma and colorectal cancer, which underscores the centrality of CDK8 in gene expression and demonstrates why CDK8 is considered an attractive and promising target for cancer treatment. Currently, how dysregulation of CDK8-CycC contributes to tumorigenesis is poorly understood. To determine the normal and dysregulated functions of these proteins, it is essential to identify the downstream effectors and upstream regulators of CDK8-CycC. Analyses of function and regulation of CDK8 have been hampered by the lack of phenotypes for CDK8 activity in multicellular organisms. We have solved this challenge by using Drosophila, which provides unparalleled sophistication in manipulating CDK8 activity in vivo. We have recently identified two important downstream transactivators of CDK8, SREBP (Sterol Regulatory Element-Binding Protein) and the EcR (Ecdysteroid Receptor), which play essential roles in regulating lipogenesis and developmental maturation, respectively. Our analyses suggest that CDK8 inhibits SREBP-dependent transcription but activates EcR-activated gene expression. Thus, the objective of this proposal is to determine how CDK8 plays fundamentally different roles in modulating the activities of SREBP and EcR. Specifically, we will elucidate how CDK8 regulates SREBP- and EcR-mediated transcription by determining the molecular mechanism underlying the interactions between CDK8 and both SREBP and EcR, and the effects of SREBP and EcR phosphorylation by CDK8 in vivo. We will also examine the physiological regulation of CDK8-CycC by the insulin-signaling pathway and analyze the role of CDK8 in coordinating lipid homeostasis and developmental timing. This study will significantly advance our understanding of how CDK8 plays distinct roles in lipogenesis and development. Because CDK8 and CycC are well conserved in eukaryotes, the molecular mechanisms that we identify in Drosophila are highly likely to provide working models in human studies. The importance of this project is highlighted by the fact that dysregulation of lipid homeostasis durin development is closely linked to diseases such as diabetes, obesity, cardiovascular diseases, and certain types of cancers.